There are two-stroke engines in which scavenging air is introduced to the combustion chamber during the downward stroke of the piston, and an air/fuel mixture is introduced to the combustion chamber after exhausting combustion gas. Known examples of these methods of introduction include feeding scavenging air to a scavenging channel connected to the crankcase and the fuel chamber, feeding an air/fuel mixture to the crankcase, and sequentially introducing these to the fuel chamber; directly introducing scavenging air to the fuel chamber, feeding an air/fuel mixture to the crankcase, and introducing the air/fuel mixture to the combustion chamber after the scavenging air; or directly introducing scavenging air and an air/fuel mixture in sequential fashion to the combustion chamber.
The flow rate of scavenging air and the air/fuel mixture must be made substantially proportional in order to prevent incomplete combustion, stabilize engine operation, and so forth; an air valve is therefore provided to the air channel for supplying scavenging air to the engine; and the air valve is operated in coordination with the throttle valve of the carburetor, which is a means for forming an air/fuel mixture.
This type of two-stroke engine is used as a power source for portable machinery and other small machines and, as a result, carburetors or air channels, air valves/throttle valve interlocking mechanisms, and other components must be installed in narrow locations requiring these components to be kept as small as possible.
Described in Japanese Patent Application Laid-open No. 10-252565 (‘565 application) as a proposal for satisfying the above-described requirements is a configuration in which a carburetor with an air intake passage for forming a portion of the air/fuel mixture channel is provided with an air passage that forms a portion of the air channel and diverges from the air intake passage inlet portion. A throttle valve in the air intake passage and an air valve in the air passage are integrated in rotary configuration. The air intake passage is connected to the engine by way of a through hole provided to an adiabatic wall, and the air passage is connected to the engine by way of a conduit pipe.
Described in Japanese Patent Application Laid-open Nos. 11-336613 and 2000-73869 (‘869 application) is a configuration in which the rotary air valve in the ‘565 application is substituted with a butterfly air valve that is integrated with the rotary throttle valve, and a conduit pipe forming a portion of the air channel is substituted with a through hole provided in the adiabatic wall.
Furthermore, described in Japanese Patent Application Laid-open No. 2002-227653 (‘653 application) is a configuration in which a carburetor with an air intake passage for forming a portion of the air/fuel mixture channel is provided with a separate air passage for forming a portion of the air channel. Both the throttle valve in the air intake passage and the air valve in the air passage are fashioned as butterfly valves and are interlocked with each other by means of a linking mechanism. The air intake passage and the air passage are connected to the engine by way of a through hole and a conduit pipe that is provided to the adiabatic wall.
The carburetors in each of the applications described above are advantageous in that they are compact. The carburetors are compact because the air passage for forming a portion of the air intake channel is provided in parallel fashion to the air intake passage for forming a portion of the air/fuel mixture channel in the carburetor main body and because an air valve is disposed in the air passage. The carburetors are made further compact because the air valve and the throttle valve are integrated with each other and do not have interlocking mechanisms.
However, as noted in each of the above described applications, the carburetors are mounted in a predetermined location on the side face of the engine with an adiabatic wall interposed therebetween. For this reason, in a configuration in which the air passage, which is made to substantially the same length as the air intake passage, is connected from the back end thereof to a predetermined location in the engine by way of an external conduit pipe as in conventional systems, positioning the conduit pipe tends to be extremely difficult depending on the location for feeding scavenging air because the space between the carburetor main body and the engine is narrow. Described in Japanese Patent Application Laid-open No. 9-268917 (‘917 application) is a configuration in which the air channel is made to diverge from the carburetor inlet portion from the air intake passage, and is connected to a predetermined position on the side face of the engine by way of an external conduit pipe. This configuration easily conforms to a variety of positional relationships between the location for feeding scavenging air and the location for feeding the air/fuel mixture in the engine because of the considerable freedom to place the conduit pipe. However, the carburetor of the ‘917 application is configured such that the throttle valve and the air valve are disposed at a right angle to each other, which complicates the interlocking mechanism and raises concerns that excessive force may be applied.
In the carburetors described in the ‘869 and ‘653 applications, the air passage and the air intake passage are connected to the engine by way of a conduit hole and a through hole formed in an adiabatic wall. Such carburetors can be used with only one type of engine, that is to say, with an engine in which a mutual match is established between the location for feeding the air/fuel mixture and the location for feeding scavenging air, and between the conduit hole and the through hole. Such carburetors are inapplicable to a large number of engines with differing positional relationships between the location for feeding the air/fuel mixture and the location for feeding scavenging air.